It has long been known that variations in the pressure of the material being molded in the mold cavity of a molding machine or cast in the cavity of a die cast machine is the most frequent cause of flash being formed in the parts being produced, short shots of material being fed to the cavity, and inconsistencies in the properties of the parts produced. With particular reference to the question of molding machines, pressure variations occur because of changes in the viscosity of the material, i.e., plastic being molded. Such changes are commonly found to be due to either temperature variations or molecular weight. Pressure variations also occur as a result of variations in machine performance due to such things as timer errors, oil temperature, hydraulic response, etc. In addition, pressure variations are attributable to operator errors wherein wrong timer settings, etc. are employed by the operator.
Most rejects, it has been found, are caused by events occurring during the filling portion, i.e., when material is being fed to the cavity, and during the packing portion, i.e., when the material is solidifying in the cavity of the molding machine, of the molding cycle. It is this part of the cycle, i.e., the filling and packing portions thereof when orientation is developed, molecular packing is induced, and peak pressures are determined. Any errors or inconsistencies in the dynamics of filling and packing result in variations in the parts produced.
As the cavity fills with material, the pressure therein will rise slowly until packing of the material begins to occur, whereupon the pressure will rise rapidly. This may be referred to as the dynamic portion of the cycle of operation of the machine. If the pressure in the cavity rises too slowly or is stopped too soon, then short shots will occur because the material is then afforded an opportunity to solidify before the cavity is entirely filled, or in the case of the mold having plural cavities, before all of the cavities are full of material. If the pressure rises too rapidly or occurs too long, then flash is likely to form on the part being produced due to overpacking.
The likelihood that pressure variations will occur causing the undesirable results described above is particularly likely where thin-walled parts, i.e., lids, containers, housings, etc. are being produced inasmuch as the production of the latter requires that the cavity be filled quickly but not be overpacked. These two requirements however are often in conflict. As a result, short shots caused by insufficient pressure, flashed parts caused by excessive pressure, and warped parts caused by overpacking or underpacking are common.
In order to eliminate the existence of undesirable pressure variations in the mold cavity, a number of different types of control systems have been proposed for inclusion in molding machines as a means of effectuating control over the latter thereby to enable the amount of pressure in the cavity of the machine to be regulated. Because of the difficulty in controlling pressure however, one approach which has been employed in the prior art is for instance the so-called volume-feeding technique wherein a limit switch or other equivalent control device is utilized to control the amount of a material fed each cycle. Moreover, the screw by means of which the material is fed into the mold is caused to move through a predetermined distance. This method works as long as the limit switch is accurate and the non-return valve operatively associated with the screw does not leak. However, these two conditions rarely exist for long.
Another approach similar to the aforedescribed volume-feeding technique, which has also been utilized heretofore is the technique of weight-feeding wherein the theoretical weight of the material needed to fill the mold completely is determined and this amount of material is caused to be fed to the mold once each cycle. Although the weight-feeding technique has been found to produce more accurate results than the volume-feeding technique, it also nevertheless suffers from a disadvantage. Namely, even a small deviation in the weight of the material fed to the mold can produce wide variations in cavity pressure.
Still another technique, which previously has sometimes been employed, involves sensing the forward velocity of the screw which is operative to cause material to be fed to the mold. The difficulty with this approach however is that a change in the forward velocity of the screw may not necessarily arise from the fact that the mold cavity has become filled with the material. Rather, this change may be attributable to the existence of other factors affecting the flow of the material intermediate the point at which the material leaves engagement with the screw and the point at which the material enters the mold cavity. More particularly, since cavity pressure is not being determined from readings taken directly within the cavity, but instead is based on readings obtained at a point remote therefrom, other factors such as a blockage in the path of flow of the material, etc., may actually be the cause for a change occurring in the forward velocity of the screw rather than the fact that the cavity has become filled with material. Thus, utilizing this technique there exists opportunities for inaccurately portraying the actual condition of the cavity pressure.
Other techniques have been employed heretofore in the prior art, however these other techniques have also suffered from one or more disadvantages. There has thus existed a need to provide a method, which did not suffer from the same disadvantages which characterize prior art methods of controlling cavity pressure, but which would be operable for purposes of automatically measuring molded part consistency and determining the proper molding machine adjustments which are necessitated in order to correct for process changes. In accord with the present invention, such a method has been provided. The latter method involves the utilization of the slope of a portion of the profile curve for the material being supplied to the mold cavity of the molding machine. More specifically, a comparison is had between the slope of a portion of the actual profile curve, the latter being determined from pressure sensings and time measurements taken during a cycle of operation of the molding machine, and a corresponding portion of the theoretical profile curve which should exist if parts having the desired characteristics are to be produced. Based on the latter comparison, any corrective adjustments which appear to be required are made to the molding machine controls.
Accordingly, it is an object of the present invention to provide a novel and improved method which is operable for purposes of determining whether the material flow conditions to the mold cavity of a molding machine which must be present in order to ensure the production of molded parts having the desired characteristics do in fact exist.
It is also an object of the present invention to provide such a method wherein there is established a portion of the actual profile curve of the pressure of the material present in a mold cavity furing a cycle of operation of a molding machine from which the slope of the aforesaid portion of the actual profile curve may be determined, and thereafter compared to the corresponding portion of the theoretical profile curve which should exist to obtain the desired product results.
It is another object of the present invention to provide such a method wherein the portion of the actual profile curve is established by taking at least a pair of pressure sensings in the mold cavity of the molding machine while simultaneously making corresponding time measurements.
A further object of the present invention is to provide such a method wherein the portion of the actual profile curve whose slope is employed for purposes of effecting a comparison therewith of the slope of a corresponding portion of the theoretical profile curve may be selected so as to lie either along the rising portion of the profile curve or along the falling portion thereof.
A still further object of the present invention is to provide such a method wherein based on the comparison of the slope of the actual profile curve with that of the corresponding portion of the theoretical profile curve, the need for effecting corrective adjustments in the molding machine controls can be established.
Yet another object of the present invention is to provide such a method which is operative such that if corrective adjustments to the molding machine controls are deemed to be required through the use of the subject method, then such adjustments can be made so as to be effective either during the same cycle of operation of the molding machine for which the portion of the actual profile curve is established, or during a succeeding cycle of operation of the molding machine.